1. Field of the Invention
The present invention relates to an image sensor. In particular, the invention relates to an image sensor that is used in facsimile machines, copiers, scanners, etc. And detects, with sensors, light that is reflected from a document surface to be read.
2. Background Art
A conventional example of the above type of apparatus is a contact type image sensor that reads information on a document with a line sensor array having same length as the document by forming an image of the document in an actual size through one to one imaging of the document pixels.
FIG. 4 is a sectional view of a conventional contact type image sensor that is disclosed in Japanese Unexamined Patent Publication No. Hie 7-147620, for example.
In FIG. 4, reference numeral 1 denotes a chassis having a sealed structure, and reference numeral 2 denotes a glass plate that is a transparent member disposed on the top surface of the chassis 1. The glass plate 2 not only transmits light coming from a light source (described later) but also holds a document to be read and prevents foreign matter from entering the chassis 1. Reference numeral 3 denotes a document that is held on the glass plate 2 and on which characters, image information, etc. including color ones are written. Reference numeral 4 denotes a platen for transporting the document 3 by rotating while pressing the document 3 against the glass plate 2. Reference numeral 5 denotes a light source provided in the chassis 1. The light source 6 is a collection of document illumination light sources having. respective spectral radiation energy distributions of red, green, or blue; for example, the light source 5 is an LED array in which red, green, and blue light-emitting diode (LED) chips are mounted on the same circuit board. The light source 5 applies light to the surface of the document 3 to be read via the glass plate 2. Reference numeral 6 denotes an erect-image, 1:1 magnification lens such as a rod lens array that is disposed below the glass plate 2 and condenses light that is reflected from the surface of the document 3 to be read. Reference numeral 7 denotes line sensor lCs that are solid-state imaging devices for imaging an image of the document 3 by detecting light that is condensed by the erect-image, 1:1 magnification lens 6. Reference numeral 8 denotes a sensor circuit board that is mounted with a plurality of line sensor ICs 7 arranged in line. Reference numeral 9 denotes a connector for input/output transfer of image readout information of the document 3 as well as power, signals, etc.
Next, a description will be made of an operation of the image sensor having the above configuration in reading a color image on the document 3.
FIGS. 5A-5D show a relationship between on-state periods of the red, green, and blue light sources 5 respectively (FIGS. 5A-5C) and an output signal of one of the line sensor ICs 7 (FIGS. 5D).
As is well known, each line sensor IC 7, that is a solid-state imaging device such as a CCD or a photodiode array, operates in an accumulation mode. That is, many photodiodes constituting a photodiode array as a line sensor accumulates charges while a red, green, or blue light source is on-state, and outputs signals after the end of the on-state of the light source. Specifically, as shown in FIG. 5D, a red output signal is output when an on-state of the red light source has finished as shown in FIG. 5A and the next light source, that is, the green light source, is on as shown in FIG. 5B.
Similarly, a green output signal is output when an on-state of the green light source has finished and charges have been accumulated during the on-period of the green light source shown in FIG. 5B and the next light source, that is, the blue light source is in an on-state as shown in FIG. 5C.
A time series output signal is produced as shown in FIG. 5D in such a manner that signals of the respective colors are sequentially output after the ends of the respective on-periods.
FIG. 6 shows a one-bit circuit configuration of a line sensor IC 5 that produces an output signal as shown in FIG. 5D.
In FIG. 6, reference symbol VB denotes a bias voltage generator, PD denotes an optical sensor, and SW1 denotes a switch that supplies the optical sensor PD with a voltage that is generated by the bias voltage generator VB. The optical sensor PD is initialized while the switch SW1 is on.
Reference symbols TR1-TR4 denote transistors that constitute a current mirror circuit for transmitting an output voltage of the optical sensor PD. A terminal voltage of the optical sensor PD is output to the gate electrode of the transistor TR1. Reference symbol SW2 denotes a switch that is part of an accumulation circuit for the optical sensor PD. While the switch SW2 is on, it transmits a gate electrode voltage of the transistor TR3 to a capacitor C and charge is accumulated there in a concentrated manner. Reference symbol TR5 denotes a transistor for outputting a voltage that has developed across the capacitor C as a result of the charge accumulation. As described above, the transistor TR5 is kept on and a current is output to a SIG terminal during an on-state period of the light source of the next color.
Reference symbol SW3 denotes a switch for supplying an output of each bit to the SIG terminal. The switch SW3 is so controlled as to prevent shooting of the output of each bit.
In the conventional image sensor having the above configuration, a red signal, that is a signal accumulated by a reflected light during a red light illumination of the document, can be taken out by readout scanning while the following green light is in an on-state. Similarly, a green signal can be taken out by readout scanning while the blue light source is on, and a blue signal can be taken out by readout scanning while the red light source is on. The red, green, and blue signals thus taken out are arranged in time-series as shown in FIG. 5D. At this time, it is desirable that the red, green, and blue signal outputs be uniform when the same white document is read. However, actually there are large variations among signal outputs of the respective colors and hence a compensating means for equalizing the signal levels is necessary.
There are other problems that high-speed reading requires amplification of the respective outputs by an image processing circuit because the absolute values of the respective signals are small, and that the amplification increases the above-mentioned variations among signals of the respective colors.
One cause of the above problems is that the spectral sensitivity characteristic of the line sensor ICs 7 is not uniform. This point will be described below in detail.
FIG. 7 is a sectional view showing an example structure of a line sensor IC formed by using a semiconductor process. In FIG. 7, reference numeral 10 denotes a p-type semiconductor substrate; 11, a LOCOS layer formed locally on the semiconductor substrate 10; 12, a polysilicon layer formed on the semiconductor substrate 10 so as to be separated from the LOCOS layer 11; 13, a n+ sensor portion formed by implanting a n-type impurity into a portion of the substrate 10 that is located between the LOCOS layer 11 and the polysilicon layer 12; and 14, a depletion layer formed along the n+ sensor portion 13. The depletion layer 14 includes space charge and generates a resulting strong electric field. Reference numeral 15 denotes a silicon borate/phosphate film (NSO/BPSG oxide film) formed on the top surfaces of the LOCOS layer 11, the polysilicon layer 12, and the n+ sensor portion 13 so as to cover those layers; 16, a first aluminum interconnection layer formed on part of the top surface of the silicon borate/phosphate film 15; 17, a second aluminum interconnection layer formed on the top surface of the first aluminum interconnection layer 16; 18, a plasma silicon oxide film (PSio film) formed on the top surfaces of the silicon borate/phosphate film 15 and the first aluminum interconnection layer 16; and 19, a protective film that is a silicon nitride film (PSG/SiN film) formed on the top surfaces of the plasma silicon oxide film 18 (PSiO film) and the second aluminum interconnection layer 17.
In the line sensor IC having the above structure, light is applied from above the protective film 19 and the n+ sensor portion 13 and the depletion layer 14 receive the light and produces electromotive force.
The protective film 19 is formed at a thickness of about 8,000 Å, and FIG. 8 shows a transmission spectrum of the protective film 19 having such a thickness. The horizontal axis represents the wavelength and the vertical axis represents the transmittance.
As is apparent from FIG. 8, the transmittance decreases (the degree of absorption increases) as the wavelength shortens. In addition to the decrease in transmittance, there are undulations due to interference. With this transmission spectrum, if the wavelengths of the red, green, and blue light sources are selected arbitrarily, for example, if they are 640 nm (red), 530 nm (green), and 460 nm (blue) that are the wavelengths of LEDs available on the market, the corresponding transmittance values are 98% (red), 85% (green), and 65% (blue). These differences in transmittance cause differences among output signals, that is, variations among outputs of the respective colors, of the image sensor.
Further, in an ordinary semiconductor process, the thickness of the protective film 19 has dispersion of about ±20%. This causes a variation of the transmission spectrum, which in turn causes a lot-by-lot sensitivity variation. The peak wavelengths of the respective colors are 640 nm, 545 nm, and 480 nm.
The reason for small absolute values of output signals, that is, low sensitivities, is that because the layers above the n+ sensor portion 13 have different refractive indices, incident light is reflected at the boundaries between those layers and does not reach the n+ sensor portion 13 and the depletion layer 14 efficiently.
Further, where a line sensor IC is formed by arranging sensors of FIG. 7 adjacent to each other at a constant pitch, outputs of the respective sensors, that is, respective bits, vary as shown in FIG. 9 because of dispersion of the area of the n+ sensor portion 13 and the depletion layer 14 of the sensor. Such dispersion occurs in the following manner. When ion implantation is performed on the substrate 10 to form the n+ sensor portion 13 and the depletion layer 14, the implantation area is defined by the LOCOS layer 11 and the polysilicon layer 12. The shape of the LOCOS layer 11 tends to vary in its formation process, as a result of which the area of the n+ sensor portion 13 becomes unstable and hence the characteristics of the n+ sensor portion 13 and the depletion layer 14 become unstable, resulting in a sensor-by-sensor variation of the electromotive force.